In wavelength-division-multiplexed (WDM) networks, dynamic reconfigurability of channel add-drop filters may be important. Reconfigurable optical add-drop multiplexers (R-OADMs) include channel add-drop filters that can discontinue dropping and/or adding a particular wavelength channel, tune the filter center wavelength to a new wavelength channel, and begin dropping data on the new wavelength channel. This operation is preferably hitless, which means that the entire reconfiguration, including disabling the filter at a first wavelength, tuning to a second wavelength, and re-enabling the filter at a new wavelength, occurs without substantial signal distortion and preferably with no bit errors in any of the wavelength channels other than those at the first and the second wavelengths (which are in any case disturbed because their transmission path is reconfigured by the operation). Channel add-drop filters capable of reconfiguring in a hitless manner are called hitless tunable, or hitless switchable.
Previous approaches to hitless tuning of integrated channel add-drop filters typically use either bypass interferometers or resonance-frequency misalignment in resonant filters. Bypass interferometers, for example, reroute the entire optical spectrum of interest in a second path, around the filter, while the filter is reconfigured to a new wavelength. These schemes typically require the actuation of a pair of optical switches in synchronization, which is a challenge for control. They are also not entirely hitless when the filter being bypassed contributes substantial out-of-band dispersion in the through port. In this case, there may be, during the switching operation, a temporary loss on the order of 1 dB or more induced in the channels adjacent to the wavelength channel being switched. Typically, these structures are also sizeable as they add an interferometer structure around the filter device, which makes them suitable only for limited replication.
High-order coupled-resonator filters, including microring resonator filters, are a promising technology for channel add-drop filters. A second approach to hitless switching, used with coupled-resonator filters, is to detune the cavities from each other, thus breaking the resonant condition required for channel-dropping operation. As a result, the channel-dropping passband of the filter, present when all cavities are aligned to the center wavelength, is disabled to a level where a very small amount of residual power is dropped (e.g., −30 dB) and most power passes in the through port. At this point, the resonators are tuned to a new wavelength. They are then brought back into alignment at a common resonance frequency to reconstitute a passband at the new wavelength, and begin dropping data on the new channel.
This approach has been used in thin-film filters, as well as in integrated optical microring resonators. On such integrated optical microring resonator 10, which includes two ring resonators 12, 14, is depicted in FIG. 1, while its switching performance 20 is depicted in FIG. 2. More specifically, FIG. 2 shows the passband amplitude of the microring resonator 10 diminished at one wavelength by the mismatching of the resonance frequencies of the two ring resonators 12, 14, the turned off microring resonator 10 tuned to a new, 1 nm longer wavelength, and the passband subsequently reconstituted at a second wavelength by matching the ring 12, 14 resonant frequencies again.
There is, however, a fundamental drawback in this approach of detuning cavities. While the drop-port amplitude response may be diminished by mismatching the resonance frequencies and the through port response recovered to substantially full transmission, the same does not hold true for the phase response. In particular, in a higher-order filter, there is typically at least one resonant cavity that is coupled to the input waveguide. If detuned so that there is no power passed to other resonators, such a cavity will act as an allpass filter in the through port. Furthermore, the coupling coefficient between the input port and the first cavity of a higher-order flat-top filter, as typically used in WDM systems, makes the bandwidth of the cavity match the desired passband width. This, in turn, sets the group delay at center wavelength of the allpass filter to about the inverse of the bandwidth, which is comparable to a bit slot of the maximum bitrate transmissible through such a filter. Therefore, the dispersion of such an allpass filter may be substantial and cause bit errors. A preferable solution would turn off both the amplitude and the phase response of a filter.
One way to turn off the amplitude and phase response of a filter is by introducing loss into a cavity. Consider, for example, a resonator coupled to an input port that has an associated internal quality factor (i.e., internal Q (or loss Q), labeled Qo) describing the losses in the cavity, and a second external quality factor (i.e., external Q, labeled Qe) defined as the quality factor associated with the decay of energy into the input port only. Then, the resonant filter is substantially turned off in both amplitude and phase response, in both the through and any drop ports, when the round-trip losses (including coupling of light to any output ports except the input port) are much higher than the power coupled to the input port in a pass (i.e., Qo<<Qe).
While this approach disables both the amplitude and phase response, it has at least two drawbacks. First, the energy coupled into the resonator in the first pass is lost to the cavity loss mechanism, and thus the approach causes a finite loss that is larger in lower finesse designs, i.e., where the free spectral range (FSR) of the cavity is smaller for a given passband width. For example, in typical 3rd to 4th-order filters with a 40 GHz passband and 2-3 THz FSR, the input coupling for a flat-top filter is on the order of 10%. Thus, in using this approach of introducing loss to quench the resonance, a broadband 10% loss is incurred across the spectrum, as the light entering on first pass is lost. Second, for high fidelity (low loss) on-state operation, it is typically necessary that the loss Q of the cavities be 10 to 100 times larger than the external Q, or than the loaded Q of the structure. On the other hand, for the resonance to be substantially turned off, the loss Q is typically required to be about 100 to 10000 times smaller than the external Q. Therefore, for good performance, it is typically necessary to vary the cavity loss by 30-40 dB between the on-state and off-state, which is a challenging prospect.
Another approach to disable the resonant response is to reduce waveguide coupling to the cavity to zero by moving the waveguides away from the resonator. This micromechanical approach typically requires considerable fabrication complexity using microelectromechanical systems (MEMS), which may also negatively affect the optimization of the optical waveguides and resonators. There is also a functional disadvantage in decoupling all waveguides from the cavity. For a well-performing on-state, the cavity round-trip loss must be much lower than the waveguide-ring coupling (i.e., Qo>>Qe). Suppose first that the cavity is lossless. If the waveguides are symmetrically decoupled, the resonator remains critically coupled and transfers all power on resonance over narrower and narrower bandwidths, with greater and greater group delay and dispersion. In practice, an asymmetry will cause either a minimum-phase through port transmission with no dispersion if the input coupling is weaker, or an allpass filter response with maximum dispersion in the through port, if the input coupling is stronger. Or, the cavity loss will dominate the coupling once the latter is made weak enough, and appropriately turn off the resonant response. However, if the loss is low, then the input coupling must typically be switched by a large contrast (30-40 dB) between its value in the on state and the required off state. Such large switching contrasts are a challenge for both switch design and reliable realization in fabrication technology. In the MEMS case, they typically require larger motion of the waveguides, and larger actuation voltages.
Referring now to FIGS. 3 and 4, a Mach-Zehnder interferometer waveguide-ring coupling 31 has been used in the past to extend the effective FSR of a resonator 30 by providing a Mach-Zehnder arm length difference equal to half the ring 32 circumference. For example, in FIG. 4, the Mach-Zehnder coupler 31 illustrated at the top of the figure has a first arm (i.e., the upper most arm in FIG. 4) between coupling points K2, K2 of length 2l2, a second arm (i.e., the top half of ring 32) again between coupling points K2, K2 of length l2, and, as such, an arm length difference of 2l2−l2=l2. As illustrated, the ring 32 circumference is equal to 2l2. In such devices 30, resonances adjacent to a resonance of interest are suppressed, thereby doubling the effective FSR.
With reference to FIG. 5, an alternative, or complementary, way to extend the FSR is the standard Vernier approach 50, where rings 52, 54 of different radii are used. This latter approach typically suffers from substantial through-port 56 dispersion at suppressed passbands near the resonance frequency of the resonator coupled to the input waveguide 58, much like the hitless tuning approach depicted in FIG. 1. FIG. 6 illustrates the spectral response 60 of a filter that employs the scheme 50 depicted in FIG. 5.
With reference to the higher-order resonant filter 70 depicted in FIG. 7, Mach-Zehnder interferometers have also been used as a wavelength-dependent loss mechanism in ring resonators in order to increase the resonator FSR. These structures 70, if used as add-drop filters, typically suffer intolerable through-port 72 dispersion in the same way as the structure 50 depicted in FIG. 5.
With reference again to FIGS. 3 and 4, a ring resonator with a Mach-Zehnder 2-point coupler has been described, for the purposes of doubling or multiplying the effective FSR of a ring resonator, in Barbarossa, Giovanni, et al., “Theoretical Analysis of Triple-Coupler Ring-Based Optical Guided-Wave Resonator,” Journal of Lightwave Technology, Vol. 13, No. 2, February 1995 (hereinafter “Barbarossa”), the contents of which are incorporated herein by reference in their entirety. For proper operation, and with reference for example to FIG. 3, equation (2) in Barbarossa and its accompanying description requires a particular relationship between i) the round trip length of the ring 32 (i.e., a first closed path) and ii) the round trip length of a second closed path formed by the longer Mach-Zehnder arm (l2 in Barbarossa and in FIG. 3) and the half of the ring that is not within the Mach-Zehnder interferometer (l3+l4 in Barbarossa and in FIG. 3). The required relationship is that the length and thus the FSR of these first and second paths beN·FSR1=M·FSR2  (1)where N and M must be relatively prime non-negative integers, i.e., integer numbers where neither divides evenly into the other. In particular, N=2 is an optimum solution for solving the FSR doubling problem. However, this configuration illustrated in FIGS. 3 and 4, which calls for relatively prime N and M, as described in Barbarossa, is not suitable for achieving hitless switching.